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Impedance For A Newbie


ksquared

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If you've taken any of these standardized tests, there is a question such as:

Human is to Foot as Horse is to ______

with the answer being Hoof.

 

Now, I absolutely can't figure out speaker impedance, despite many attempts.

 

It reminds me of my combine driving days, I almost took out a fence post, and the supervisor calmly said, "OK, back it up and take it out REAL SLOW."

 

So if somebody can go REAL SLOW, it would be greatly appreciated.

 

I understand a speaker specification 8 ohm @ 99 db, and another one has 4 ohm @ 89 db.  But what I can't understand is:  

4 ohm @ 89 db is to 8 ohm @ _____?  

Is the 8 ohm speaker a higher db rating or a lower db rating?  I might understand that 4 ohm is more efficient somehow, but from what I pretty much don't understand, that is only if your amp can "double down" which is beyond my understanding.

 

I've seen a spec for the XA25 amp, where it says "outputs 25Wpc into 8 ohms, or 50Wpc into 4 ohms or 100Wpc into 2 ohms!"  So it somewhat makes sense, as the resistance is cut in half, the output is doubled.  Which would tie into the "if your amp can double down" phrase.  But I then don't understand why any amp wouldn't "double down," as if it sends X Wpc into 8 ohm, it should be able to send 2X Wpc into 4 ohm.

 

So any help is greatly appreciated.

 

 

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Speaker sensitivity is not directly tied to impedance.

 

Impedance is the resistance to current flow.  Impedance is DC resistance + AC "resistance" (a simplification)

Water makes a great analogy.  Voltage is Pressure.  Current is flow.  Impedance/Resistance is nozzle size. 

More pressure increases flow through the same nozzle (restriction).  Less restriction (Impedance) allows more flow (Current) at the same pressure (Voltage).

Ohms law says Current = Voltage/Resistance.  I=V/R (V=IxR)

Power = Current x Voltage.  P=IxV  Substituting Ohms law gets  P=(VxV)/R (or P=IxIxR)

 

Two speaker have the same sensitivity at one watt (one of the rating standards), but one is 16 ohms and the other is 8. 

If the same voltage is applied, say 2.83V, the power into 8 ohms is P=(2.83x2.83)/8 = 1 watt.  For the other one it is P=(2.83x2.83)/16 = 0.5 watts. 

Power = I^2xR.  1 watt = I^2 x 8.  I^2 = 1 watt/8ohms.  I = sqrt(1/8) = 0.354 amps

Power = I^2xR.  .5 watt = I^2 x 16.  I^2 = .5 watt/16ohms.  I = sqrt(.5/16) = 0.177 amps

Half the amps and half the power into 16 ohms at the same voltage.  .

Lower impedance "allows" more current (and thus power) at the same voltage.   It is linear, so half the impedance doubles the power.

This is why car speaker are 4 ohms or less.  Battery voltage is limited to ~13V, but it can surge 300 to 400 amps. 

This relationship is important because power amps want to be voltage sources (a source of voltage with unlimited current capacity).  As you turn up the volume knob, you are really turning up the voltage applied to your speakers.  So, what happens if you turn up the voltage to 20 volts? 

Assuming your amp is capable, Power = (20v x 20v)/8 ohms = 50 watts.  At 4 ohms it would be 100 watts. 

 

However, there is no such thing as a Voltage Source.  Nothing, especially power amps, are that linear.  One prime reason is that current flow causes heat and heat causes resistance to increase.  That heat must also be pulled away from the transistors to keep them from burning up, too.  Removing heat takes relatively expensive large heat sinks or even fans.  My sub amps are Acurus A250s, rated at 250 wpc into 8 ohms.  They test at 300 watts into 8 ohms and 480 into 4 ohms.  They are conservatively rated and still don't double their rated output into 4 ohms. 

 

Speaker impedance is never a flat curve, but varies wildly sometimes, but the enclosures and crossovers help control the loudness in spite of the variations and modern power amps tolerate that foolishness well, most of the time. 

 

One thing to watch in speaker ratings is way it is rated.  Is it 1 watt at 1 meter? Or is it 2.83V at 1 meter? 

2.83 V is 1 watt into 8 ohms, but it is 2 watts into 4 ohms. 

Doubling the power increases output 3 dB! 

 

FYI, 10x the power is 10 dB, or twice as loud to the ear. 

https://www.crownaudio.com/en/tools/calculators

 

 

 

 

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Thank you JohnA.  I do agree too, an awesome answer!

 

This does help greatly too.  I can understand that the Acurus A250s as a real-world example, technically can't "double down" from 8 ohms to 4 ohms, because of the physics of the amp.  But they are able to supply a significantly higher watts at 4 ohms, because the impedance is cut in half (significant).

 

But I'm still left wondering, if you put the amp to power a 4 ohm 89db speaker, and set the output to 89 db, would it then power the 8 ohm speaker at 86db (89db - 3db) at that same set power output?

 

So is it then 0.5 watts into the 4 ohm 89db speaker, giving 89 db, and 0.5 watts into the 8 ohm 89 db speaker then gives 86 db (since it would need 1 watt to output 89db)?

 

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I've often wondered what the effects are of running different taps on an amp equipped with multiple ohm outputs... I mean "go ahead and try it" but is there a general characteristic change in sound when running an 8ohm speaker on the 4ohm tap? 

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3 hours ago, ksquared said:

But I'm still left wondering, if you put the amp to power a 4 ohm 89db speaker, and set the output to 89 db, would it then power the 8 ohm speaker at 86db (89db - 3db) ... 

 

I see what you are trying to do, but it doesn’t work that way - read John’s first sentence again. 

 

Simply put, impedance varies with frequency. Loudspeakers are really not “8 ohms” or “4 ohms”. The rating is “nominal”, based on its measured impedance curve. If the measured response rides much below 6 ohms, or has several or more 3 or 4 ohm dips in its response, it will typically be given a 4 ohm rating. This is why the sensitivity rating is so important - all we want to know is what the output is using 2.83 volts, which gives us an apples to apples comparison between loudspeakers. 

 

From the amplifier side, if the loudspeaker presents an overall lower impedance, it will generate more output - but not all amplifier sections can do this gracefully, which is why many overheat and shutdown.

 

https://www.audioholics.com/loudspeaker-design/loudspeaker-sensitivity

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2 hours ago, Schu said:

I've often wondered what the effects are of running different taps on an amp equipped with multiple ohm outputs... I mean "go ahead and try it" but is there a general characteristic change in sound when running an 8ohm speaker on the 4ohm tap? 

 

A solid state amp has a very low output impedance (high damping factor) - a tube amp doesn’t. With a tube amp, you want to match the nominal impedance of the loudspeaker to the corresponding output tap from the output transformer. If you know the low frequencies are hovering in that 4-6 ohm area, moving to the four ohm tap will improve the sound of the bass (improved damping). However, there is a cost - you will sacrifice a bit of your amplifier’s output. 

 

“There is no free lunch.” - DJK

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19 hours ago, ksquared said:

..............

So is it then 0.5 watts into the 4 ohm 89db speaker, giving 89 db, and 0.5 watts into the 8 ohm 89 db speaker then gives 86 db (since it would need 1 watt to output 89db)?

 

 

No, since the POWER is equal into both speakers, both will produce the same loudness if their sensitivity is equal.  To get equal POWER the  voltage and current will both change. 

 

If you attach 2 speakers of equal sensitivity to an amp, (let's use a receiver with a speaker A/B switch) and adjust the  amp's output voltage to some number and then switch from the 4 ohm speaker to the 8 ohm speaker, the 8 ohm speaker will be 3 dB quieter.  That's because the impedance of the 8 ohm speaker restricts current flow to 1/2, thus cutting power input to 1/2.  Note, our experiment deliberately held VOLTAGE constant. 

 

We could also hold current constant, but would have to adjust voltage to do it.  It takes twice the voltage to push 1 amp through 8 ohms as is does 4 ohms. 

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